Transmission systems often use a twisted pair to carry signal information as a difference signal. In such systems, it is useful to generate a mean (i.e., common mode) of the voltages that can vary from rail-to-rail, with a difference signal that can vary independently, but is bounded. The impedance presented to the inputs should be high, e.g., greater than 1MΩ. Additionally, the speed of response should be high, e.g., much greater than a 10 MHz bandwidth.